CHAPTERS ON THE STARS. 651 



is approached, in which time there may be as great an increase in two 

 or three days as there formerly was in a month. The diminution of 

 light is generally slower than the increase. The magnitude at corres- 

 ponding times in different periods may be very different. Thus, as 

 we have already remarked, Omicron Ceti is ten times as bright at some 

 maxima as it is at others. The periods also, so far as they have been 

 made out, vary more widely than those of stars of the other type. 



The idea has sometimes been entertained that these variations of 

 light are due to a revolution of the star on its axis. A very little 

 consideration will, however, show that this explanation cannot be valid. 

 However bright a star might be on one side, or however dark on the 

 other, any one region of its surface would be visible to us half the time 

 and a change of brightness from different degrees of brilliancy on 

 different sides would be gradual and regular. 



It is not impossible that the variability may be in some way 

 connected with the action of a body revolving round the star. This 

 seems to be the case with Eta Aquilae. The radial motion of this object 

 shows the existence of a dark body revolving round it in the same period 

 as that of the star's variation. 



From what has been said, it will be seen that, although a sharp 

 line cannot be drawn, there seems to be some distinction between the 

 stars of short and long periods. The number of stars which have been 

 known to belong to the first class is quite small, only about fifteen, 

 all told. On the other hand, there are still left some stars having a 

 period less than ten days, which are otherwise not distinguishable from 

 the Omicron Ceti type. It seems quite likely that the variations in the 

 periods of these stars are, in some way, connected with the revolution of 

 bright or dark bodies round them. 



They also vary more widely than those of stars of the other two 

 types. This might easily happen in the case of stars really variable 

 through a cycle of changes going on in consequence of the action of 

 interior causes. 



The periodic stars of short period, which have not been recognized 

 as of the Algol or Beta Lyrge type, form an interesting subject of study. 

 Although the separation between them and the stars of long period is 

 not sharp, it seems likely to have some element of reality in it. But 

 no conclusions on the subject can be reached until the light-curves 

 of a large number of them are carefully drawn; and this requires an 

 amount of patient and accurate observation which cannot be carried out 

 for years to come. 



SUSPECTED VAEIATIONS IN THE COLOR OF STARS. 



The question whether certain stars vary in color without materially 

 changing their brightness has sometimes been raised. This was at 



